Apache deployment

Hello,
We are lauching a really big application that will interacts with TV Campaigns, Radio, etc.
The front-end applications, is written with Phalcon, and then we have a service tier running with java.
Our devopts team is configuring the apaches with fpm. However, during the stress test we are not having the results that we are expecting.

We're running on an 8 cores, 15 gb server (AWS c4.2xlarge).. if we use
pm.max_children = 50
we get 90rps, but our load is 12 with no errores. If we set up a higher number (e.g.: 1024) we get a load of 40, no errors as well. If we set it to 10, we get a sane load (6, 7) but we start getting 408 errors, even without traffic.

As a side note, we also included and removed apc without significant changes.